Amplifier speaker drive current sense

ABSTRACT

A class-D amplifier includes measurement of speaker current via the low-side drive transistors of the amplifier. In one embodiment, a class-D amplifier includes two high-side transistors, two low-side transistors, a first sense resistor, a second sense resistor, and a sigma delta analog to digital converter (σΔ ADC). The two high-side transistors and two low-side transistors are connected as a bridge to drive a bridge tied speaker. The first sense resistor is connected between a first of the low-side transistors and a low-side reference voltage. The second sense resistor is connected between a second of the low-side transistors and the low-side reference voltage. The ΣΔ ADC is coupled to the bridge to measure voltage across the first sense resistor and the second sense resistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Indian Provisional PatentApplication No. 201641023194, filed Jul. 6, 2016, entitled “HighlyAccurate Current Sense Architecture For Speaker Current Sense In AudioSmart Amplifiers,” which is hereby incorporated herein by reference inits entirety.

BACKGROUND

Class-D audio amplifiers are switch mode amplifiers that switch at ahigh frequency to produce a rectangular waveform at the amplifier'soutput. Class-D amplifiers may be much more efficient that linear audioamplifiers, and as a result may employ smaller power supplies andeliminate heat sinks. Accordingly, class-D amplifiers may significantlyreduce overall system costs, size, and weight relative to linearamplifiers of equivalent power.

Some class-D amplifiers use a pulse width modulator (PWM) to generatepulses that vary in width with the audio signal's amplitude. The pulsesmay switch output transistors of the amplifier at a fixed or variablefrequency. Some class-D amplifiers may rely upon other types of pulsemodulators, such as pulse density modulators. The rectangular waveformgenerated by the class-D amplifier is filtered to remove thehigh-frequency carrier waveform and reconstruct the audio waveform,which can be used to drive a speaker and produce sound.

Some class-D amplification systems include circuitry to model andmonitor speaker performance. Such systems use the speaker performanceinformation to optimize amplifier output and protect the speaker. Theseamplification systems may be referred to as “smart amplifiers.”

SUMMARY

A class-D amplification system that includes measurement of speakercurrent via the low-side drive transistors of the amplifier is disclosedherein. In one embodiment, a class-D amplifier includes two high-sidetransistors, two low-side transistors, a first sense resistor, a secondsense resistor, and a sigma delta analog to digital converter (ΣΔ ADC).The two high-side transistors and two low-side transistors are connectedas a bridge to drive a bridge tied speaker. The first sense resistor isconnected between a first of the low-side transistors and a low-sidereference voltage. The second sense resistor is connected between asecond of the low-side transistors and the low-side reference voltage.The sigma delta analog to digital converter is coupled to the bridge tomeasure voltage across the first sense resistor and the second senseresistor.

In another embodiment, a speaker drive and current measurement systemincludes a transistor bridge and a ΣΔ ADC. The transistor bridgeincludes two high-side switching transistors, a first low-side switchingtransistor, a second low-side switching transistor, a first senseresistor, and a second sense resistor. The first sense resistor connectsthe first low-side switching transistor to a low-side reference voltage.The second sense resistor connects the second low-side switchingtransistor to the low-side reference voltage. The ΣΔ ADC is coupled tothe bridge. The ΣΔ ADC includes a first digital to analog converter(DAC) and a second DAC. The first DAC is dedicated to measurement ofvoltage across the first sense resistor. The second DAC is dedicated tomeasurement of voltage across the second sense resistor.

In a further embodiment, an audio amplifier includes a driver and acurrent monitoring system. The driver is configured to drive a speaker.The current monitoring system is configured to measure the currentoutput by the driver. The driver includes two high-side transistors, twolow-side transistors, a first sense resistor, and a second senseresistor. The two high-side transistors and two low-side transistors areconnected as a bridge to drive the speaker. The first sense resistor isconnected between a first of the low-side transistors and a low-sidereference voltage. The second sense resistor is connected between asecond of the low-side transistors and the low-side reference voltage.The current monitoring system includes a ΣΔ ADC coupled to the driver tomeasure voltage across the first sense resistor and the second senseresistor. The ΣΔ ADC includes a differential amplifier, a first switch,a second switch, a first DAC, and a second DAC. The first switch is toswitchably connect one of a positive side of the first sense resistor ora negative side of the second sense resistor to a first input of thedifferential amplifier. The second switch is to switchably connect oneof a positive side of the second sense resistor or a negative side ofthe first sense resistor to a second input of the differentialamplifier. The first DAC is dedicated to measurement of voltage acrossthe first sense resistor. The second DAC is dedicated to measurement ofvoltage across the second sense resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows a schematic diagram for a class-D amplification system inaccordance with various examples;

FIG. 2 shows a schematic diagram for a delta-sigma analog-to-digitalconverter suitable for use in a smart amplifier in accordance withvarious examples; and

FIG. 3 shows a flow diagram for a method for measuring speaker drivecurrent flowing from a class-D amplifier in accordance with variousexamples.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, different companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In the following discussion and inthe claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to . . . . ” Also, the term “couple” or “couples” isintended to mean either an indirect or direct wired or wirelessconnection. Thus, if a first device couples to a second device, thatconnection may be through a direct connection or through an indirectconnection via other devices and connections. The recitation “based on”is intended to mean “based at least in part on.” Therefore, if X isbased on Y, X may be a function of Y and any number of other factors.

Some smart amplifiers measure the current driving the speaker to producereal-time speaker diagnostics. In such amplifiers, the currentmeasurements are used to determine load conditions in real-time, andused in diagnostics that aid in maximizing power delivery to the loadand in improving sound pressure level. Unfortunately, the currentmeasurement techniques employed in conventional smart amplifiers aresubject to a variety of problems. In some conventional smart amplifiers,sense transistors are added to mirror the currents flowing in the lowside drive transistors. The addition of the sense transistors andassociated circuitry requires significant circuit area and additionalpower, and provides limited measurement accuracy because using the sensetransistors negative currents cannot be sensed.

Other conventional smart amplifiers insert a sense resistor in serieswith the speaker, and measure the voltage drop across the senseresistor. Because the signal to common mode swings at the sense resistorcan be high (e.g., as high as 1:140, for 14V supply, 50 m Ohms Senseresistor and 2 A peak load), current measurement using a load senseresistor is limited, and when employed may require a floating supply toaccommodate common mode variations and analog level shifters, both ofwhich increase circuit area. Moreover, the additional components in thesignal path limit achievable accuracy.

Embodiments of the present disclosure provide improved currentmeasurement accuracy with low circuit area. In the amplifier circuitdisclosed herein, a sense resistor is inserted between each of thelow-side drive transistors and the low-side reference voltage source. Anovel sigma-delta analog-to-digital converter (ΣΔ ADC) measures thevoltage across the sense resistors to determine the current flowing inthe speaker. The ΣΔ ADC includes two digital-to-analog converters(DACs), two sets of input resistors and two sets of integrator feedbackcapacitors, where one DAC, one set of input resistors and one set offeedback capacitors corresponds to each of the sense resistors. BothDACs are always active to eliminate DAC settling time issues. Switchingcircuitry in the ΣΔ ADC selects which of the sense resistors to monitorbased on the drive transistor gate drive signals. Control circuitryconnects a sense resistor coupled to a last activated low-side drivetransistor to the ΣΔ ADC. If both low-side drive transistors areactivated, the switching circuitry maintains connection of a currentlyselected sense resistor to the ΣΔ ADC.

FIG. 1 shows a schematic diagram for a class-D amplification system 100in accordance with various examples. The class-D amplification system100 includes class-D amplifier control circuitry 102, drive transistors104 and 106 (illustrated as 104-1, 104-2, 106-1, and 106-2), currentsense resistors 108-1 and 108-2 (collectively sense resistors 108), andΣΔ ADC 110. A speaker 114 is included in FIG. 1 for completeness. Thespeaker 114 is the load driven by the amplification system 100 and isnot part of the amplification system 100.

The class-D amplifier control circuitry 102 receives as input an analogor digital signal AIN. The signal AIN may be an audio signal that is tobe amplified for driving the speaker 114. The class-D amplifier controlcircuitry 102 may include modulation circuitry (e.g., pulse widthmodulation circuitry), operational amplifiers, filters, comparators,transistor drivers (e.g., metal oxide field effect transistor (MOSFET)gate drivers), and other components and circuits suitable for producingsignals to control the drive transistors 104 and 106. The class-Damplifier control circuitry 102 may also include circuitry (e.g., amicrocontroller or digital signal processor) that adjusts the signalscontrolling the drive transistors 104, 106 responsive to the measurementvalues provided by the ΣΔ ADC 110. For example, current measurementsprovided by the ΣΔ ADC 110 may be used to determine temperature and/orexcursion of the speaker 114, which can be used to optimize speakerdrive under working conditions.

The class-D amplifier control circuitry 102 produces the signals 112that control the activation and deactivation of the drive transistors104 and 106. The drive transistors 104 and 106 may be power MOSFETs orother transistors. The drive transistors 104 and 106 are connected toform bridge. The drive transistors 104-1 and 104-2 are connected on thehigh-side of the bridge (i.e., the drive transistors 104-1 and 104-2 arethe high-side drive transistors. The drive transistors 106-1 and 106-2are connected on the low-side of the bridge (i.e., the drive transistors106-1 and 106-2 are the low-side drive transistors. As shown in FIG. 1,the speaker 114 is connected across the bridge formed by the drivetransistors 104 and 106 (i.e., the speaker is bridge-tied). Activationof transistors 104-1 and 106-2 produces current flow through the speaker114 in a first direction, and activation of transistors 104-2 and 106-1produces current flow through the speaker 114 in a second direction

The sense resistor 108-1 is between the low-side drive transistor 106-1and the low-side reference voltage source. For example, in theembodiment of FIG. 1, a first terminal (positive side terminal) of thesense resistor 108-1 is connected to the source terminal of the low-sidedrive transistor 106-1, and a second terminal (negative side terminal)of the sense resistor 108-1 is connected to ground. Similarly, in FIG.1, a first terminal (positive side terminal) of the sense resistor 108-2is connected to the source terminal of the low-side drive transistor106-2, and a second terminal (negative side terminal) of the senseresistor 108-2 is connected to ground.

The class-D amplifier control circuitry 102 controls the drivetransistors 104 and 106 for operation in “low-side recycle mode.” Inlow-side recycle mode, at least one of the low-side drive transistors106 is active (i.e., turned on) at any point in time. Accordingly,embodiments of the class-D amplifier control circuitry 102 can measurecurrent flowing in the low-side drive transistors 106 to measure thecurrent flowing in the speaker 114. The current flowing in the low-sidedrive transistors 106 also flows through the sense resistors 108.Embodiments of the class-D amplification system 100 measure the voltagedrop across the sense resistors 108 to determine the current flowing inthe speaker 114.

The ΣΔ ADC 110 is connected to both terminals of each of the senseresistors 108 for measurement of the voltage across each sense resistor108. Because the current flowing in the speaker 114 changes direction asthe two different pairs of drive transistors 104, 106 are switched, theΣΔ ADC 110 senses current flowing in both of the sense resistors 108 andcombines the current sense information. The digital output of the ΣΔ ADC110 includes a measurement of the current flowing the speaker 114 (as ameasurement of voltage across the sense resistors 108), and is providedto the class-D amplifier control circuitry 102 for use in controllingthe drive transistors 104 and 106 to optimize operation of the speaker114.

FIG. 2 shows a schematic diagram for an embodiment of the ΣΔ ADC 110.The ΣΔ ADC 110 includes DACs 202 (shown in FIG. 2 as DACs 202-1 and202-2), switches 208 (shown in FIG. 2 as switches 208-1 and 208-2),switch control circuitry 214, integrators 216, 218, and 220, summingamplifier 222, and quantizer 212. The digital output of the quantizer212 is provided to the two DACs 202. The DACs 202 are connected to theinputs of the switches 208, with the two signals making up thedifferential output of each DAC connected to different switches 208.Both of the DACs 202 are always active (i.e., always on) to eliminatethe requirement for fast settling when switching from measurement of onecurrent path (i.e., the paths of current flow through sense resistors108) to the other. Each of the DACs 202 is dedicated to one of thecurrent paths. For example, in the embodiment of FIG. 2, DAC 202-1 isdedicated to measurement of the current flowing in sense resistor 108-2,and DAC 202-2 is dedicated to measurement of the current flowing insense resistor 108-1. Each DAC 202 may be trimmed to compensate for adifference in gain (e.g., mismatch between the current sense resistors108) between the two current paths.

The integrator 216 includes a differential amplifier 210, capacitors 204(shown in FIG. 2 as capacitors 204-1 and 204-2), capacitors 206 (shownin FIG. 2 as capacitors 206-1 and 206-2), input resistors 224 (shown inFIG. 2 as input resistors 224-1 and 224-2), and input resistors 226(shown in FIG. 2 as input resistors 226-1 and 226-2). Thus, theintegrator 210 includes two pairs of feedback capacitors and inputresistors. One pair of feedback capacitors corresponding to each DAC 202and sense resistor 108. The capacitors 204 are connected to providefeedback from a first output of the differential amplifier 210 to afirst input of the differential amplifier 210. Similarly, the capacitors206 are connected to provide feedback from a second output of thedifferential amplifier 210 to a second input of the differentialamplifier 210. Each of the switches 208 is connected to one of theinputs of the differential amplifier 210. In the embodiment of FIG. 2,the switch 208-1 is connected to the input of the differential amplifier210 receiving feedback via capacitors 204, and the switch 208-2 isconnected to the input of the differential amplifier 210 receivingfeedback via capacitors 206. Each of the terminals of the current senseresistors 108 is connected to an input terminal of one of the switches208. One terminal of each of the sense resistors 108 is connected to adifferent one of the switches 208, such that in a first position of theswitches 208 (the switches 208 are shown in the first position in FIG.2) the two terminals of the sense resistor 108-1 are connected to theinputs of the differential amplifier 210, and in a second position ofthe switches 208 the two terminals of the sense resistor 108-2 areconnected to the inputs of the differential amplifier 210. Similarly,the outputs of the DACs 202 are connected to the switches 208 such thatin the first position of the switches 208 the two outputs DAC 202-2 areconnected to the inputs of the differential amplifier 210, and in thesecond position of the switches 208 the two outputs DAC 202-1 areconnected to the inputs of the differential amplifier 210.

The switch control circuit 214 controls the operation of the switches208. The switch control circuit 214 sets the position of the switches208 based on drive state of the drive transistors 104 and 106. At leastsome of the signals 112 that control the drive transistors 104 and 106,or equivalent signals, are provided to the switch control circuit 214.For example, in some embodiments, the drive signals 112-1 and 112-2 thatcontrol the low-side drive transistors 106 (or equivalents thereof) maybe provided to the switch control circuit 214. If the control signals112 indicate that drive transistor 106-1 is active and drive transistor106-2 is inactive, then the switch control circuit 214 may set theswitches 208 to connect the sense resistor 108-1 to the differentialamplifier 210. If the control signals 112 indicate that drive transistor106-2 is active and drive transistor 106-1 is inactive, then the switchcontrol circuit 214 may set the switches 208 to connect the senseresistor 108-2 to the differential amplifier 210. If the signals 112indicate that both drive transistors 106 are active, then the switchcontrol circuit 214 may maintain the switches 208 in a current state(i.e., the switch control circuit 214 may not change the position of theswitches 208 if both drive transistors 106 are active).

The output of the integrator 216 is further integrated by integrators218 and 220. Some embodiments may not include the integrators 218 and220, and the techniques disclosed herein may be used with any modulatorarchitecture (including but not limited to feedforward architecture,feedback architectures, with or without resonators, etc.). The outputsof the integrators 216, 218, and 220 are summed by the summing amplifier222 and provided to the quantizer 212 for generation of digital outputvalues that are fed to the DACs 202 and the class-D amplifier controlcircuitry 102.

The class-D amplification system 100, including the ΣΔ ADC 110, providesa number of advantages over conventional smart amplifiers that includecurrent sensing. Embodiments of the system 100 may provide significantlymore accurate current measurement than conventional systems because thesystem 100 provides negative current sensing and is not subject to thelarge swings in common voltage found in conventional systems. Thecircuit area required to implement current sensing in the system 100 isgreatly reduced relative to conventional systems because only a singleΣΔ ADC is employed, while conventional systems may require multipleamplifier blocks or charge pumps. The reduced common mode voltage swingprovided by the system 100 also tends to reduce circuit area becausefewer capacitors may be needed to accommodate the swing.

FIG. 3 shows flow diagram for a method 300 for measuring speaker drivecurrent flowing from a class-D amplifier in accordance with variousexamples. Though depicted sequentially as a matter of convenience, atleast some of the actions shown can be performed in a different orderand/or performed in parallel. Additionally, some embodiments may performonly some of the actions shown. The method 300 may be performed byembodiments of the amplification system 100 that includes a ΣΔ ADC 110with two DACs.

In block 302, the DAC associated with measurement of current flowing ina first current sense resistor is enabled. For example, the DAC may beDAC 202-1 of the ΣΔ ADC 110 that is associated with measurement ofcurrent flowing in the sense resistor 108-2. The DAC may be enabled(e.g., powered and operating) for an entire duration of operation of theamplification system 100.

In block 304, the DAC associated with measurement of current flowing ina second current sense resistor is enabled. The DAC associated withmeasurement of current flowing in a second current sense resistor isdistinct from the DAC associated with measurement of current flowing ina first current sense resistor. For example, the DAC may be DAC 202-2 ofthe ΣΔ ADC 110 that is associated with measurement of current flowing inthe sense resistor 108-1. The DAC may be enabled (e.g., powered andoperating) for an entire duration of operation of the amplificationsystem 100.

In block 306, the switch control circuit 214 is monitoring the state ofthe drive transistors 104 and/or 106. The switch control circuit 214 maymonitor the state of the drive transistors 104 and/or 106 by monitoringthe state of control signals 112. If the switch control circuit 214determines that both speaker drive paths are active (e.g., both drivetransistors 106-1 and 106-2 are turned on), then, in block 308, theswitch control circuit 214 maintains the current state of the switches208. That is, the current sense resistor 108 currently connected to thedifferential amplifier 210 via the switches 208 remains connected to thedifferential amplifier 210 while both low-side drive transistors 106 areactive.

If the switch control circuit 214 determines that both speaker drivepaths are not active, then, in block 310, the switch control circuit 214determines whether a first drive path is active. For example, the switchcontrol circuit 214 determines whether low-side drive transistor 106-2is active. If the switch control circuit 214 determines that the firstdrive path is active, then the switch control circuit 214 sets theswitches 208 to connect the sense resistor 108 of the first drive pathto the differential amplifier 210 in block 312. For example, the switchcontrol circuit 214 may set the switches 208 to connect the senseresistor 108-2 to the differential amplifier 210 and disconnect thesense resistor 108-1 from the differential amplifier 210.

If, in block 310, the switch control circuit 214 determines that thefirst drive path is not active, then, in block 314, the switch controlcircuit 214 determines whether a second drive path is active. Forexample, the switch control circuit 214 determines whether low-sidedrive transistor 106-1 is active. If the switch control circuit 214determines that the second drive path is active, then the switch controlcircuit 214 sets the switches 208 to connect the sense resistor 108 ofthe second drive path to the differential amplifier 210 in block 316.For example, the switch control circuit 214 may set the switches 208 toconnect the sense resistor 108-1 to the differential amplifier 210 anddisconnect the sense resistor 108-2 from the differential amplifier 210.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A class-D amplifier, comprising: two high-sidetransistors and two low-side transistors connected as a bridge to drivea bridge tied speaker; a first sense resistor connected between a firstof the low-side transistors and a low-side reference voltage; a secondsense resistor connected between a second of the low-side transistorsand the low-side reference voltage; and a sigma delta analog to digitalconverter (ΣΔ ADC) coupled to the bridge to measure voltage across thefirst sense resistor and the second sense resistor.
 2. The class-Damplifier of claim 1, wherein the ΣΔ ADC comprises: a first digital toanalog converter (DAC) dedicated to measurement of voltage across thefirst sense resistor; and a second DAC dedicated to measurement ofvoltage across the second sense resistor.
 3. The class-D amplifier ofclaim 2, wherein the first DAC and the second DAC are trimmed tocompensate for a difference between the first sense resistor and thesecond sense resistor.
 4. The class-D amplifier of claim 2, wherein thefirst DAC and the second DAC are continually active while operating theclass-D amplifier.
 5. The class-D amplifier of claim 1, wherein the ΣΔADC comprises: a differential amplifier; a first switch to switchablyconnect one of a positive side of the first sense resistor or a negativeside of the second sense resistor to a first input of the differentialamplifier; a second switch to switchably connect one of a positive sideof the second sense resistor or a negative side of the first senseresistor to a second input of the differential amplifier.
 6. The class-Damplifier of claim 5, further comprising a control circuit configured tocontrol operation of the first switch and the second switch, the controlcircuit configured to: monitor signals controlling the low-sidetransistors; drive the first switch and the second switch to connect thefirst sense resistor to the differential amplifier based on the signalsindicating activation of the first low side transistor and deactivationthe second low side transistor; drive the first switch and the secondswitch to connect the second sense resistor to the differentialamplifier based on the signals indicating activation of the second lowside transistor and deactivation the first low side transistor; andmaintain a current state of the first switch and the second switch basedon the signals indicating activation of both the first low sidetransistor and the second low side transistor.
 7. The class-D amplifierof claim 5, wherein the ΣΔ ADC comprises: a first feedback capacitorconnected from a first output of the differential amplifier to a firstinput terminal of the first switch; a second feedback capacitorconnected from the first output of the differential amplifier to asecond input terminal of the first switch.
 8. The class-D amplifier ofclaim 7, wherein the ΣΔ ADC comprises: a third feedback capacitorconnected from a second output of the differential amplifier to a firstinput terminal of the second switch; a fourth feedback capacitorconnected from the second output of the differential amplifier to asecond input terminal of the second switch.
 9. A speaker drive andcurrent measurement system, comprising: a transistor bridge comprising:two high-side switching transistors; a first low-side switchingtransistor; a second low-side switching transistor; a first senseresistor that connects the first low-side switching transistor to alow-side reference voltage; and a second sense resistor that connectsthe second low-side switching transistor to the low-side referencevoltage; a sigma delta analog to digital converter (ΣΔ ADC) coupled tothe bridge, the ΣΔ ADC comprising: a first digital to analog converter(DAC) dedicated to measurement of voltage across the first senseresistor; and a second DAC dedicated to measurement of voltage acrossthe second sense resistor.
 10. The system of claim 9, wherein the ΣΔ ADCcomprises: a differential amplifier; a first switch to switchablyconnect one of a positive side of the first sense resistor or a negativeside of the second sense resistor to a first input of the differentialamplifier; and a second switch to switchably connect one of a positiveside of the second sense resistor or a negative side of the first senseresistor to a second input of the differential amplifier.
 11. The systemof claim 10, further comprising a control circuit configured to controloperation of the first switch and the second switch, the control circuitconfigured to: monitor signals controlling the first and second low-sideswitching transistors; drive the first switch and the second switch toconnect the first sense resistor to the differential amplifier based onthe signals indicating activation of the first low side switchingtransistor and deactivation the second low side switching transistor;drive the first switch and the second switch to connect the second senseresistor to the differential amplifier based on the signals indicatingactivation of the second low side switching transistor and deactivationthe first low side switching transistor; and maintain a current state ofthe first switch and the second switch based on the signals indicatingactivation of both the first low side switching transistor and thesecond low side switching transistor.
 12. The system of claim 10,wherein the ΣΔ ADC comprises: a first feedback capacitor connected froma first output of the differential amplifier to a first input terminalof the first switch; a second feedback capacitor connected from thefirst output of the differential amplifier to a second input terminal ofthe first switch.
 13. The system of claim 12, wherein the ΣΔ ADCcomprises: a third feedback capacitor connected from a second output ofthe differential amplifier to a first input terminal of the secondswitch; a fourth feedback capacitor connected from the second output ofthe differential amplifier to a second input terminal of the secondswitch.
 14. The system of claim 9, wherein the first DAC and the secondDAC are continually active while the bridge is driving the speaker. 15.The system of claim 9, wherein the first DAC and the second DAC aretrimmed to compensate for a difference between the first sense resistorand the second sense resistor.
 16. An audio amplifier, comprising: adriver configured to drive a speaker; and a current monitoring systemconfigured to measure the current output by the driver; wherein thedriver comprises: two high-side transistors and two low-side transistorsconnected as a bridge to drive the speaker; a first sense resistorconnected between a first of the low-side transistors and a low-sidereference voltage; a second sense resistor connected between a second ofthe low-side transistors and the low-side reference voltage; wherein thecurrent monitoring system comprises a sigma delta analog to digitalconverter (ΣΔ ADC) coupled to the driver to measure voltage across thefirst sense resistor and the second sense resistor, the ΣΔ ADCcomprising: a differential amplifier; a first switch to switchablyconnect one of a positive side of the first sense resistor or a negativeside of the second sense resistor to a first input of the differentialamplifier; a second switch to switchably connect one of a positive sideof the second sense resistor or a negative side of the first senseresistor to a second input of the differential amplifier; a firstdigital to analog converter (DAC) dedicated to measurement of voltageacross the first sense resistor; and a second DAC dedicated tomeasurement of voltage across the second sense resistor.
 17. The audioamplifier of claim 16, further comprising a control circuit configuredto control operation of the first switch and the second switch, thecontrol circuit configured to: monitor signals controlling the low-sidetransistors; drive the first switch and the second switch to connect thefirst sense resistor to the differential amplifier based on the signalsindicating activation of the first low side transistor and deactivationthe second low side transistor; drive the first switch and the secondswitch to connect the second sense resistor to the differentialamplifier based on the signals indicating activation of the second lowside transistor and deactivation the first low side transistor; andmaintain a current state of the first switch and the second switch basedon the signals indicating activation of both the first low sidetransistor and the second low side transistor.
 18. The audio amplifierof claim 16, wherein the ΣΔ ADC comprises: a first feedback capacitorconnected from a first output of the differential amplifier to a firstinput terminal of the first switch; a second feedback capacitorconnected from the first output of the differential amplifier to asecond input terminal of the first switch.
 19. The audio amplifier ofclaim 18, wherein the ΣΔ ADC comprises: a third feedback capacitorconnected from a second output of the differential amplifier to a firstinput terminal of the second switch; a fourth feedback capacitorconnected from the second output of the differential amplifier to asecond input terminal of the second switch.
 20. The audio amplifier ofclaim 16, wherein: the first DAC and the second DAC are continuallyactive while the driver is active; and the first DAC and the second DACare trimmed to compensate for a difference between the first senseresistor and the second sense resistor.